1. Field of the Invention
The present invention generally relates to an image reading device that reads image data of an original document with an image sensor and an image forming apparatus, such as a digital copying machine, a laser beam printer, a facsimile machine, etc., that forms an image based on the image data output from the image reading device and more particularly, to a control of reading operation of a reference white plate for obtaining shading data in the image reading device having a sheet-through mechanism.
2. Discussion of Background
An image reading device used in a widespread digital copying machine, a scanner, or other similar image forming apparatus, may employ a method of reading an image of an original document by a two-dimensional scanning in which a charge-coupled device (CCD) line sensor is moved in a sub-scanning direction perpendicular to a main scanning line relatively to an original document illuminated by a lamp.
The image reading device employing such an image reading method in which the CCD line sensor is moved relatively to the original document, generally uses two types of image reading mechanisms. One type of image reading mechanism may be referred to as an image reading system moving mechanism in which an image of an original document set on an original document setting table (e.g., a contact glass) in a stationary state is read by moving an image reading system. Another type of image reading mechanism may be referred to as a sheet-through mechanism in which an original document is fed by an original document feeding device (hereinafter simply referred to as a “document feeder”) so as to pass through a stationary image reading system including a CCD line sensor. An image of an original document is read by the stationary image reading system while moving the original document relative to the image reading system.
Recently, an image reading device often has both of the above-described two types of image reading mechanisms. Hereinafter, a description will be made to an image reading device that reads an image of an original document by both of the two types of image reading mechanisms.
The image reading device having both of the two types of image reading mechanisms reads an image of an original document by a common image reading system. When performing an image reading operation in the sheet-through mechanism, the image reading operation is performed in a condition that a movable image reading system is held stationary at a predetermined image reading position provided at a part of an original document conveying path in a document feeder. In the image reading device having both of the two types of image reading mechanisms, it is not desirable that an output of image read by the image reading system moving mechanism is different from that of image read by the sheet-through mechanism or an output of image read by either of the two image reading mechanisms changes with time. Therefore, in order to stabilize outputs of read images, an image reading system reads a reference white plate before reading an image of an original document to obtain a reference white level or shading data based on the data of read reference white plate. A white level of image data of a read original document is corrected using the shading data.
In the image reading system moving mechanism, following a reading operation of a reference white plate arranged between a home position of an image reading system and a position where an original document is set on a contact glass, an image reading operation of the original document is performed.
In the sheet-through mechanism, as described above, an image reading system reads an image of a moving original document fed by a document feeder in a condition that the image reading system is held stationary at a predetermined image reading position. Therefore, when reading the same reference white plate also used in the image reading system moving mechanism, the image reading system needs to be moved to the position where the reference white plate is arranged. This results in adding a step of moving the image reading system contrary to the aim of eliminating the necessity of moving the image reading system in the sheet-through mechanism. Accordingly, in order not to reduce the advantage of high throughput in the sheet-through mechanism allowing an image reading device to read original documents consecutively, it may be desired to reduce a whole period of time required for performing the reading operation of the reference white plate as much as possible.
In the above-described image reading device, a carriage carrying the image reading system is driven to move by a stepping motor. Generally, an image reading operation is controlled to be performed when the movement of the carriage changes to a constant velocity movement after being accelerated from a stop position of the carriage. Subsequently, the movement of the carriage is controlled to be decelerated after the image reading operation. Such a control operation is performed in both of an image reading operation of an original document and a reading operation of a reference white plate.
FIG. 7 is a velocity chart illustrating a control of movement of a carriage and a control of reading operation of a reference white plate according to a background art. With regard to a timing of reading a reference white plate according to a background art, a reading operation of the reference white plate is performed in an area indicated by a reference character “A” in FIG. 7 in which the carriage moves at a constant velocity and vibrations of an image reading system carried by the carriage are the weakest after the carriage is driven to move by a stepping motor and is accelerated. Alternatively, in order to reduce a time of reading the reference white plate as much as possible, the reading operation of the reference white plate is performed in an area indicated by a reference character “B” in FIG. 7 in which the carriage is accelerated, with some disadvantages such as vibrations of the image reading system.
Recently, demands have been increasing for an image reading performance satisfying both high speed and high image quality such as an image reading speed of 40 sheets/minute and a resolution of 1200 dpi, for example, in a color image reading device. In such a color image reading device, if a reading operation of a reference white plate is performed in the area “A” in FIG. 7, it takes time until vibrations of an image reading system become weak, and a start of the reading operation is delayed, thereby increasing a whole period of time required for performing the reading operation.
If the reading operation of the reference white plate is performed in the area “B” in FIG. 7, the reading operation can be started earlier than in the area “A”. However, there may be a high possibility of occurrence of error in data of the read reference white plate due to the vibrations generated when the carriage is driven to move from a stop position. As a result, it may be hard to obtain appropriate shading data.